The use of gamma ray detectors in general, and positron emission tomography or PET detectors in particular, is growing in the field of medical imaging. In PET imaging, a radiopharmaceutical agent is introduced into an object to be imaged via injection, inhalation, or ingestion. After administration of the radiopharmaceutical, the physical and bio-molecular properties of the agent will cause it to concentrate at specific locations in the human body. The actual spatial distribution of the agent, the intensity of the region of accumulation of the agent, and the kinetics of the process from administration to its eventual elimination are all factors that may have clinical significance. During this process, a positron emitter attached to the radiopharmaceutical agent will emit positrons according to the physical properties of the isotope, such as half-life, branching ratio, etc.
The radionuclide emits positrons, and when an emitted positron collides with an electron, an annihilation event occurs, wherein the positron and electron are destroyed. Most of the time, an annihilation event produces two gamma rays (at 511 keV) traveling at substantially 180 degrees apart.
By detecting the two gamma rays, and drawing a line between their locations, i.e., the line-of-response (LOR), one can retrieve the likely location of the original disintegration. While this process will only identify a line of possible interaction, by accumulating a large number of those lines, and through a tomographic reconstruction process, the original distribution can be estimated. In addition to the location of the two scintillation events, if accurate timing (within few hundred picoseconds) is available, a time-of-flight (TOF) calculation can add more information regarding the likely position of the event along the line. Limitations in the timing resolution of the scanner will determine the accuracy of the positioning along this line. Limitations in the determination of the location of the original scintillation events will determine the ultimate spatial resolution of the scanner, while the specific characteristics of the isotope (e.g., energy of the positron) will also contribute (via positron range and co-linearity of the two gamma rays) to the determination of the spatial resolution the specific agent.
The above described detection process must be repeated for a large number of annihilation events. While each imaging case must be analyzed to determine how many counts (i.e., paired events) are required to support the imaging task, current practice dictates that a typical 100-cm long, 18FDG (fluoro-deoxyglucose) study will need to accumulate several hundred million counts. The time required to accumulate this number of counts is determined by the injected dose of the agent and the sensitivity and counting capacity of the scanner.
The scanner will acquire counts in a three-dimensional mode, meaning that each line emitted from the object and crossing two detector elements is potentially detected. Thus, for example, the two emission points A and B disposed as indicated in FIG. 1 have a different probability of being detected. The point B in the middle of the center of the axial line has the maximum chance of being detected as it has the largest solid angle, including the two legs of the annihilation event. Point A supports a smaller angle, while a point right at the edge of the axial FOV would have a minimal chance of being detected. This analysis is used to determine the overall scanner sensitivity, which has a triangular shape typical of a completely open 3D scanner, as shown on the right-hand side of FIG. 1.
A special case of this situation is when, by electronics design or otherwise, the largest angles are cut off from the analysis since large oblique angles of incidence may present some challenges for some reconstruction algorithms. In this case, the typical triangle would be flattened at some point related to the cut-off angle.
This triangular axial sensitivity profile is useful as the typical 100 cm PET studies require more than one axial FOV of the scanner. Several bed positions need to be added. Usually, 50% overlap between steps is optimal as it creates a flat sensitivity over the central portion of the image, as show in FIG. 2, which shows the scanner FOV with respect to the total length requested by the user.
A repeat of the same FOV with a 50% overlap per scan is illustrated in FIG. 3.
A fixed step size will likely create an undershoot or an overshoot of the targeted length. Since under-sampling the region of interest is not an option, the scanner is likely to take an extra step to cover the entire area. In this case, the overshoot area, shown at the bottom of FIG. 3, is small. In practice, the imaging system may only present achievable length by forcing the user to step through the possible discrete length. Either way, the total length of the PET image will slightly overshoot the length the clinician would have selected.
This effect will be exacerbated in larger axial FOV scanners, as shown in FIG. 4, in which the overshoot area is visibly more important. This effect is also proportionally increased when the requested area of interest is shorter, as shown in FIG. 5.
An alternative approach is to design the system with a continuous bed motion in which the speed profile and the total length scanned can be controlled more precisely. However, continuous bed motion is not always available is all systems and definitely adds complexity to data acquisition and reconstruction.
Thus, as illustrated in FIG. 3-5, in a step-and-shoot system, a significant amount of time and counts can be spent outside of the area of interest, meaning that the same imaging resources could have been spent more efficiently.
Conventional PET scanners have an axial FOV between 16 and 22 cm to cover a typical 100 cm length. With an overlap of 50%, 8 to 12 steps are typically required, with possibly 8-12% of imaging resources being spent outside the area of interest. Other studies, like lung or head-neck imaging, are generally 30 to 50 cm long, where the waste of imaging resources can be as high as 20-30%.